Combined cycle for forming and annealing

ABSTRACT

Capitalizing on the unique feature of our induction heating workcell that permits rapid and controlled heating and cooling of a workpiece within a wide temperature range allows us to combine manufacturing into a single heating cycle to save time, energy, capital, touch labor, and factory space. For example, we can combine superplastic forming (SPF) with annealing two operations that occur at temperatures that differ by about 150-200° F. (85-105° C.) or more, to produce quality parts. We mill parts flat to simplify their machining, form them to complex curvature, and anneal them.

REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application based upon U.S.patent application Ser. No. 08/451,247, filed May 26, 1995, which is acontinuation-in-part application based upon U.S. patent application08/406,349, filed Mar. 17, 1995, now U.S. Pat. No. 5,700,995, which is adivisional application based upon U.S. patent application 08/151,433,filed Nov. 12, 1993, now U.S. Pat. No. 5,420,400, which is acontinuation-in-part application based upon U.S. patent application Ser.No. 07/777,739, filed Oct. 15, 1991, now U.S. Pat. No. 5,410,132. Thepresent application also is related to U.S. Pat. No. 4,622,445.

TECHNICAL FIELD

The present invention relates to energy efficient and timesaving methodsinvolving in an induction heating workcell for forming and annealingflat, milled parts to produce aerospace parts of compex curvature.

BACKGROUND ART

Under certain conditions, some materials can be plastically deformedwithout rupture well beyond their normal limits, a property calledsuperplasticity. This property is exhibited by certain metals andalloys, within limited ranges of temperature and strain rate. Forexample, titanium and its alloys are superplastic in the temperaturerange from about 1450-1850° F. (785-1010° C.).

Superplastic forming (SPF) is a fabrication technique that relies onsuperplasticity. A typical SPF process involves placing one or moresheets of metal or plastic in a die, heating the sheets to an elevatedtemperature within the superplastic range, and superplastically formingthe sheet(s) at the SPF temperature. Generally, a differential formingpressure from a gas manifold is used to stretch the sheet(s) into thedesired shape against the die surface(s). This forming process can becalled blow molding insofar as it uses differential pressure to form thematerial. The differential pressure is selected to strain the materialat a strain rate that is within its superplastic range. The followingpatents are illustrative of SPF processes and equipment:

    ______________________________________    PATENT  TITLE              ISSUE DATE    ______________________________________    3,920,175            Method of SPF of Metals with                               November 18, 1975            Concurrent Diffusion Bonding    3,927,817            Method for Making Metallic                               December 23, 1975            Sandwich Structures    3,605,477            Precision Forming of Titanium                               September 29, 1971            Alloys and the Like by Use of            Induction Heating    4,141,484            Method of Making a Metallic                               February 27, 1979            Structure by Combined Flow            Forming and Bonding    4,649,249            Induction Heating Platen for                               March 10, 1987            Hot Metal Working    4,117,970            Method for Fabrication of                               October 3, 1978            Honeycomb Structures    5,024,369            Method to Produce  June 18, 1991            Superplastically Formed            Titanium Alloy Components    ______________________________________

We incorporate these patents by reference.

One advantage of SPF is the forming of complex shapes from sheet metalwhile reducing the time and eliminating the waste of milling, producingconsiderable cost saving. In addition, the SPF process is generallyapplicable to single and multisheet fabrication. For multisheetfabrication, SPF is combined with joining processes, such as diffusionbonding, brazing or laser welding, to produce complex sandwichstructures. One advantage of the SPF process is lighter, lower costparts with fewer fasteners. A single part can replace the complexassembly currently required using conventional manufacturing operations.Common applications of SPF include the manufacture of parts foraircraft, missiles, and spacecraft.

In a typical prior art SPF process for titanium or its alloys, the sheetmetal is placed between dies, at least one of which has a contouredsurface corresponding to the shape of the product. The dies, are placedon platens which are heated, generally using embedded resistive heaters.The platens heat the dies to about 1650° F. (900° C.). Because thetitanium will readily oxidize at the elevated temperature, an inert gas,such as argon, surrounds the die and workpiece. The dies heat the sheetmetal to the temperature range where the sheet metal is superplastic.Then, under applied differential pressure, the sheet metal deformsagainst the contoured surface.

The platens and dies have a large thermal mass. They take considerabletime and energy to heat and are slow to change their temperature unlessdriven with high heat input or with active cooling. To save time andenergy, they must be held near the forming temperature throughout aproduction run (i.e., the production of a number of parts using the samedies). The raw sheet metal must be inserted onto the dies, and formedparts removed, at or near the elevated forming temperature. The hotparts must be handled carefully at this temperature to minimize bending.Within the SPF range, the SPF metals have the consistency of taffy, sobending can easily occur unless the operators take suitable precautions.

As described to some degree in U.S. Pat. No. 4,622,445 and U.S. Pat. No.5,410,132 we have discovered an improvement for an SPF process couplingthe use of ceramic dies with inductive heating. With our inductivelyheated SPF press or workcell, we can heat preferentially the sheet metalworkpiece with induction heating without heating the platens or diessignificantly and can use the ceramic dies as an insulator to hold theinduced heat in the part. We can stop the heating at any time and cancool the part relatively quickly even before removing it from the die.We do not waste the energy otherwise required to heat the large thermalmass of the platens and dies. We do not force the press operators towork around the hot dies and platens. With our inductive heatingworkcell, we also save time and energy when changing dies to set up tomanufacture different parts because the dies and platen aresignificantly cooler than those in a conventional SPF press. We shortenthe operation to change dies by several hours. Therefore, the inductionheating process is an agile work tool for rapid prototyping or low rateproduction with improved efficiency and versatility.

U.S. Pat. Nos. 3,920,175 and 3,927,817 describe typical combined cyclesfor SPF forming and diffusion bonding. Diffusion bonding is anotoriously difficult and temperamental process that has forced many SPFfabricators away from multisheet manufacturing or to "clean room"production facilities and other processing tricks to eliminate thepossibility of oxidation in the bond. Oxides foul the integrity of thebond. In addition, diffusion bonds are plagued with microvoids which aredifficult to detect nondestructively, but, if present, significantlydiminish the structural performance of the joint. Diffusion bonding alsois a time consuming process. The part typically must be held at elevatedtemperature and elevated pressure (about 400 psi) for several hours. Forexample, in U.S. Pat. No. 3,920,175, the diffusion bonding operationtakes five hours at 1650° F. (900° C.), making the forming/bondingoperation six hours. In U.S. Pat. No. 3,927,817, diffusion bondingoccurs prior to forming, still requires four to five hours, and forces asix hour bonding/forming cycle at 1650° F. (900° C.) for the entireperiod. Typically a hot press diffusion bonding process for commontitanium alloys used in aerospace applications will require over eighthours at 2500 psi and 800° C. (1472° F.), about six hours at 400 psi and900° C. (1650° F.), or about two hours at 250-300 psi and 950° C. (1742° F.). Producing this heat and pressure for this length of time isexpensive.

The methods of the present invention capitalize on the ability of theinduction heating press to rapidly change the temperature of the part onwhich it operates. Conventional processing requires a significantlyhigher investment in capital equipment and requires the use of separateequipment maintained at the different temperatures to produce parts thatrequire multiple, elevated temperature manufacturing operations. In ourinvention, we combine heating cycles to reduce hand labor, capitalequipment cost, and energy consumption. We combine SPF withbeta-annealing of titanium or its SPF alloys and might also includeother heat treatments in the same cycle.

We focus heat on the part we are forming using an induction heater. Wehold the part within insulating ceramic dies that are transparent to thetime-varying magnetic field that our induction heater produces. Wesignificantly reduce cycle time in manufacturing modern aerospace partsby combining cycles and, here, we reduce the production cost bysimplifying the milling operation and completing the part quickly byforming the flat preform to the finished configuration.

SUMMARY OF THE INVENTION

By combining operations into a single, sequential heat cycle with rapidtemperature adjustments between the manufacturing operations, we use ourinduction heating process in agile manufacturing operations that save ussignificant time while consuming far less energy and far less investmentcapital than conventional manufacturing operations. Furthermore, weenhance safety by reducing or eliminating the need for operators totransfer hot work-in-process between processes. We save floor space bycombining operations within one workcell. We achieve these benefits bythe generic process of combining several elevated temperaturemanufacturing operations into a single heating cycle for the part as wecomplete the several combined operations at significantly differenttemperatures in our induction heating workcell through the focusing andcontrol of heating using induction heating. Induction heaters cause thesheet metal workpiece to heat but not the ceramic dies, leading to muchshorter heating and cooling cycles as compared to the prior arttechnique of using resistive heaters and metal dies.

A preferred manufacturing operation combines forming, and generallysuperplastic forming, with subsequent beta-annealing and optionallyadditional heat treatment. This combined cycle process permits machiningof flat sheets of titanium (6Al-4V) ELI alloy with subsequent forming inour induction heating press and then beta-annealing to increase thefracture strength in the final product. The annealing involves heatingthe part to about 1850-1950° F. for a predetermined time whilemaintaining the elevated forming pressure to achieve geometric stabilityin the completed part followed by controlled cooling to around ambienttemperature at a rate between about 30-85° F. /minute to achieve thedesired optimum microstructure (or, at least to about 500° F.). Then thepart is reheated to about 1400° F. and held for an extended period toachieve the desired stabilization anneal heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical, idealized pressure-temperature profile for thecombined inductive heating cycle of the present invention.

FIG. 2 A-F is a schematic cross-section of a preferred five-sheet packand retort, illustrating the typical assembly process.

FIG. 3 A-F is a schematic cross-section of a preferred four-sheetpack-retort, illustrating the typical assembly process.

FIG. 4 is an idealized pressure-temperature profile for a combined cyclein which brazing precedes the forming operation.

FIG. 5 is idealized pressure-temperature profile for a combined cycleinduction heating operation involving forming, beta-annealing andstabilization annealing.

FIG. 6 is a schematic sectional view of a typical flat, milled partpositioned in one workcell and ready for forming and annealing inaccordance with a method of the present invention.

FIG. 7 is a schematic sectional view of the part of FIG. 6 after formingand annealing.

BEST MODE CONTEMPLATED FOR CARRYING OUT THE INVENTION

The present invention relates to improved manufacturing operations inwhich we can rapidly change the temperature of the workpiece to permitthe combination of operations into a single heating cycle using asingle, versatile manufacturing workcell, particularly our inductionheating process. The basic induction heating process is described inconsiderable detail in our prior U.S. Pat. Nos. 4,622,445 5,599,472,5,410,132, and 5,420,400 which we incorporate by reference.Nevertheless, we will briefly describe that equipment and its basicoperation. A more detailed description will then follow with respect tothe method of milling flat parts followed by forming to complexcurvature.

I. THE INDUCTION HEATING WORKCELL

Fundamentally, the induction heating workcell is a manufacturing toolcapable of providing controlled heating and pressure to a workpiecemounted in the workcell to accomplish manufacturing operationsincluding, e.g., forming, superplastic forming, brazing, diffusionbonding, consolidating, curing, welding, bonding, annealing, or heattreating. The workpiece can be isolated from the surroundingenvironment, usually by confining the workpiece within a metal envelopeor bagging system, particularly like that described in U.S. patentapplication 08/341,779, 5,599,472 or U.S. patent application Ser. No.08/469,604 entitled "Method for Achieving Thermal Uniformity inInduction Processing of Organic Matrix Composites or Metals," which weincorporate by reference.

The workcell includes a pair of dies 100 & 102 (FIG. 6) preferablyformed from a dielectric thermally insulating (i.e., relatively poorheat conducting), castable ceramic. The workpiece 104 is positionedbetween the dies. The base of the workcell includes four columns atcorners of the dies. The upper or lower die conveniently may be rigidlymounted on the column through a jackscrew and threaded bore arrangementwhile the other die might float freely on the columns with bushings orother suitable means sliding over the outer dimensions of the columns.If free floating, then, appropriate nuts can be carried on the threadedportion of the jackscrews to stop motion of the dies in the desiredlocation. In this way, the workcell can open to allow the workpiece tobe positioned within the dies or withdrawn and can clamp on theworkpiece to apply some of the pressure that we desire in themanufacturing operation or operations that we will complete in theworkcell. Typically the dies are carried in metal strongbacks thatactually are carried on the columns since the metal strongbacks are moredurable. The strongbacks also allow the changing of dies relativelyeasily so that we can perform different operations requiring differentdies with agility (i.e., significantly reduced machine setup or changedelays). The strongback provides a stiff, flat surface backing theceramic die to transfer the load applied through the columns evenly tothe die. The strongback should have sufficient rigidity to keep theceramic die from bending and cracking and, therefore, should hold thedie rigid to a surface variation or tolerance under the appliedpressure/stress of +0.003 in/ft² of die surface.

Each die contains a portion of the induction coil which we use to heatthe workpiece. The induction coil, therefore, actually surrounds theworkpiece and includes the several, spaced, parallel, straight coilsegments 106 we embed in the dies plus peripheral jumper segments. Theembedded coil segments are typically copper tubing. The copper carriesthe current which we use to create the oscillating, magnetic field thatfunctions as our heat source. Coolant, usually water, circulates in thetubing to control the temperature of the tubing and the surround die.Although illustrated as circular pipes, the shape of the tubing is notcritical. We also use rectangular channels.

At the edges of the dies, fittings on the tubing transition the embeddedcoil to insulated copper jumpers enclosed within a fluid-tight, flexiblejacket. Here, the jumpers carry the current while the jacket carries thecoolant. This jumper arrangement permits a range of travel between thedies of 3 inches or more. Other jumper/coolant arrangement might also beused.

The coil connects with a coil driver which supplies the power we desireto accomplish heating of the part. Typically, the power is anywhere upto about 400 kW at between about 3-10 kHz to create an oscillatingmagnetic field around the workpiece of high field strength andsubstantial uniformity. Temperature uniformity in the workpiececorresponds with the uniformity of the field to a great extent so we tryto create a field that at any moment in time is the same throughout thevolume of space within the coil. A uniform temperature in the workpieceensures that all portions of the part are properly heated when weinitiate a manufacturing operation, particularly forming or SPF. Also, auniform field means that all portions of the workpiece will heatuniformly because they experience the same driver.

We prefer tool inserts within the ceramic dies to achieve even greaterflexibility in operation by further reducing the mass of the dieportions that are replaced during tool changes. Such tool inserts areceramic blocks having faces shaped to the final part configuration andbacking surfaces are designed for a friction fit alignment withcorresponding surfaces fabricated into the corresponding die.

Each die is supported around its periphery with a compression frame 101which we usually mold from phenolic resin, dielectric beams. Each diealso includes preloaded, reinforcing tie rods 103 that are held betweenthe corresponding frames in both the lateral and transverse directions.In this way, the compression frame functions to apply a compressiveforce to the ceramic. Furthermore, the frame defines the border of thedie when the ceramic is cast.

Next, we will discuss a typical manufacturing operation in our workcell.

II. A COMBINED SPF AND BRAZING CYCLE

Combining superplastic forming with metallurgical joining of theindividual sheets of a multisheet pack, generally using brazing, in onethermal operation provides significant manufacturing cost advantagewhile reliably producing complex multisheet SPF parts. Typically,forming precedes brazing. Replacing diffusion bonding that isconventional in the art with a metallurgical bond or braze jointeliminates the uncertainties of structural integrity that worrymanufacturers who rely on diffusion bonding. We can form a metallurgicalbond quickly and reliably at lower pressures and significantly shorterprocessing times than a corresponding diffusion bond. We can inspect thebraze joint or metallurgical bond to verify its quality.

As shown in FIG. 1, the idealized temperature-pressure cycle for apreferred SPF/brazing process of the present invention includes fiveprocessing zones. In Zone I, the multisheet pack is loaded to the SPFworkcell (i.e., "press") and is heated inductively to the superplasticforming temperature for the pack. In Zone II, the press's gas manifoldand gas control system creates differential pressure as the drivingforce in the pack to form the SPF parts with a preselected core cellgeometry. In this step, the pack typically has sheets that are expandedto form a web, truss, or "honeycomb" structure between outer face sheetswhich may be formed or that may remain flat, as appropriate for theparticular part being manufactured. Those skilled in the art of SPFmanufacturing will understand how particular parts can be designed andlaid up into the packs with suitable gas zones to produce the desiredfinal shape. We illustrate two multisheet pack-retort lay-ups in FIGS. 2and 3.

In Zone II, the SPF process also defines the location of braze joints inthe finished part. When assembling the pack, braze alloy is affixed toone or more of the sheet metal sheets in these areas. A braze alloy isselected that has a melting point higher than the SPF formingtemperature of the pack sheet metal so that the forming and brazingoperations can be sequential in a single thermal cycle. Generally we usea braze alloy having a braze temperature about 150° F. (85° C.) abovethe SPF forming temperature when making titanium or titanium alloyparts.

After the SPF operation is complete, in Zone III, the pack is heatedinductively to the higher melting point temperature of the braze alloy,and is held there briefly (Zone IV) to allow the alloy to melt and flowin the area of the braze joint. Finally, the heating is stopped and thepart is allowed to cool (Zone V) below the SPF temperature, and thefinished part is removed from the press.

Inductive heating is a significant advantage for this SPF/brazingoperation because it permits rapid heating and cooling. Oscillatingcurrent in the induction coil within the ceramic dies creates atime-varying magnetic field that couples electromagnetically with themetallic pack to heat the pack rapidly by inducing eddy currents in thepack. The ceramic dies are a natural refractory material that has a lowheat transfer coefficient and are an effective thermal insulation; theysurround the pack. The induction only creates eddy currents in sheets onthe outside of the metal. The pack and the dies hold the heat producedby induction in these sheets. Energy is not wasted in heating a hugethermal mass otherwise typical for conventional SPF presses. While thepack might have a temperature of 1650° F. (900° C.), the ceramic dieswill only heat to on the order of 100-400° F. (37-205° C.) and,preferably 100-200° F. (37-93° C.) remote from the forming surface.Hence, there are significant energy savings and the workplace is a saferand more pleasant environment. The forming operation can proceed morequickly between tool changes or setup as occurs frequently in agileaerospace production where parts are manufactured at low rates (e.g.,20/mon) with short production runs. Our workcell is particularly wellsuited to rapid prototyping.

The details of the forming/brazing operation of the present inventionwill next be discussed with reference to forming a four sheet titaniumhoneycomb part using Ti 6Al-4V alloy sheet metal and TiCuNi (70: 15: 15)braze alloy. Such a part includes three sheets (10, 12, 14; FIG. 2) thatconstitute the core material and two face sheets 16 and 18 that sandwichthe core material. The five sheets constitute "a pack."

The three sheets (10, 12 and 14) of the core material are welded 20 (orjoined in any other appropriate manner) in selected locations to jointhem in a pattern appropriate to form the desired core cell geometry(i.e., web, truss, honeycomb, etc.). Typically the core material isabout 0.025 in thick sheet metal. We often use resistance or laserwelding and may use stopoff material (i.e., a release agent) whereverthe core material sheets are not welded together. We prefer to dowithout stopoff. The core material is welded around essentially itsentire periphery, leaving appropriate gas inlets for the forminggas/differential pressure.

Braze alloy ribbon (0.001 in thick) 22 is spot welded to the appropriatelocations on the face sheets 16 and 18 where the core material andrespective face sheet will contact in the finished part. Again, we canapply stopoff material, if desired, at the remaining portions of theface sheets.

The core material is sandwiched between the face sheets with the brazealloy ribbons in contact with the corresponding locations on the corematerial where we will form braze joints. The face sheets may be weldedsubstantially around their periphery (leaving the gas inlets, of course)to complete the pack. The sheet metal stock is oversized to accommodatethe welds and to allow trimming to produce the final, finished part.

A vacuum line into the pack allows evacuation or pressurizing of thevolume between the face sheets where the core materials lay. Thedifferential pressure zones necessary for the SPF forming are also laidout at this time, as those skilled in the art will understand.

The pack is sandwiched, in turn, between two 1008 steel sheets 24 and 26that are welded or otherwise sealed around their periphery (leavingappropriate gas inlets 108 FIG. 6!) to form a retort. As an alternativeto welding, we can use the sheet sealing system that we described incopending U.S. patent application Ser. No. 08/341,779. In that system,we machine or form grooves around the periphery of the sheets and seat asealing gasket in the groove so that the dies create a compression sealon the pack when they engage it. The sheets of the retort are coated ontheir inner sides with a release agent like boron nitride to keep thepart from sticking to the retort. The retort sheets can be selected sothat their Curie temperature is high enough to facilitate the formingand brazing operations. The Curie temperature is a measure of themagnetic permeability of the ferromagnetic material used as thesusceptor, so a judicious selection of the retort sheet material isnecessary. For titanium, we typically use 1008 steel or copper. Foraluminum, we use 1100 aluminum alloy.

The weld around the retort sheets defines an internal volume or cavityfor the pack. At least one gas line permits evacuating or pressurizingthis volume which surrounds the pack. Typically we purge both the retortcavity and the pack cavity with argon gas or another suitable inert gas.

The retort usually is coated with boron nitride and EVERLUBE, ceria,graphite, mixtures thereof, or another appropriate release agent on itsouter surfaces to ensure that the retort does not stick to the dies.

We load the retort into the SPF press and connect the gas lines 108.With induction heating, we rapidly heat the retort and pack until theyreach SPF forming temperature of about 1650° F. (900° C.). With ourpress, we can heat the part at a rate of about 165° F./min (92° C./min)so this heating step (Zone I in FIG. 1) takes about 10 min. During thisheating stage, we prefer to purge the pack cavity with argon.

At the forming temperature, we ramp up the pressure (FIG. 1, Zone II) toapply the differential pressure that will form the part. Thedifferential pressure and the forming temperature are maintained for theproper time (typically 40 min) while the part stretches superplasticallyinto the configuration of the die and the core material expands todefine the desired core material cell geometry and the locations of thebraze joints where a face sheet and core material sheet sandwich a brazealloy ribbon. The differential pressure ensures that this sandwichbrings the core material into intimate contact with the braze alloyribbon.

With SPF forming complete, we increase the power to the induction coilto induce an increase in temperature in the retort and the temperatureis ramped up (FIG. 1, Zone III) quickly at a rate of about 100° F./min(55° C./min) to the braze temperature (melting point) of the brazealloy, here about 1850° F. (1010° C.). The rate of gain should be asrapid as possible, while maintaining thermal uniformity. Radiant heattransfer from one hot spot on the part to a cooler neighboring locationand convection in the cavities helps to maintain this temperatureuniformity, should any discontinuities occur.

Once the braze alloy melts (i.e., the melting point is reached), thetemperature can be brought down immediately, because sufficient flowwill occur in the braze joint to ensure a quality bond. We turn off theinduction coils at this point and achieve a cooling rate (FIG. 1, ZoneV) of about 60° F./min (33° C./min). In this Zone, we reduce thedifferential pressure as well. When the retort cools below thesuperplastic forming range (and usually to about 600° F. (315° C.) orbelow), we remove the retort from the die. At 600° F. (315° C.), thepart will have good strength and the risk of warping or other distortionwhen the retort contacts ambient air is significantly reduced.

We cut away the retort leaving the finished part for trimming andfitting.

The ceramic die faces usually are coated with a release agent as well tokeep the retort from sticking to the die.

While our detailed example is about Ti-6Al-4V ELI alloy, titanium andits other SPF alloys (like standard Ti-6-4 or Ti-6-2-2-2-2), aluminumand its SPF alloys, stainless steel, or other SPF metals can readily beused in the combined SPF/brazing thermal cycle.

Although the braze alloy in our example melts at or above theβtransition (a.k.a. "β-transus") for titanium, the parts we wish to makerequire a β-transition annealing, so this fact is not a disadvantage. Itmay be a concern, however, with the fabrication of other parts. Thisfact along with performance issues will drive the braze alloy selection.Silver alloys are lower melting (˜1750° F. (954° C.)) and are analternative braze when avoiding , βtransus is desirable. Such silverbraze alloys are typically foils about 0.002-0.004 in thick. We willdiscuss β-annealing in greater detail in section III.

We can also braze a metal matrix composite (MMC) film or sheet (i.e., acladding) to the inner or outer surfaces of the face sheets of the packin this combined forming / brazing cycle. For example, we might form apack with two Ti-6Al-4V sheets for the core material brazed to two 0.031in thick titanium MMC sheets that sandwich the core material and that inturn are bonded to outer face sheets of Ti-15-3-3-3. We are developingsuch Ti MMCs with 3M and we describe such MMCs in our quarterlytechnical report for September, 1992, through November, 1992, underContract No. MDA972-90C-0018, Subcontract GS00347-KAS for ARPA. Makingthese products in the induction cycle we propose here improves thembecause they are exposed to elevated temperatures for a shorter time,thereby reducing interaction between the reinforcing fibers with thematrix metal.

Of course, the brazing step can precede the forming step asschematically illustrated with the temperature-pressure profile of FIG.4. There, the solid line represents the temperature profile and the" - - - 0 - - - 0 - - - " line the pressure. The temperature scale is onthe left and the pressure scale on the right. If brazing precedesforming, stopoff material needs to be applied everywhere at metal:metalinterfaces where braze joints will not be located. Brazing first,however, can improve the fatigue characteristics. Forming a braze filletbefore SPF expansion seems to strengthen the finished part.

While this description has discussed only the forming of a braze jointusing a braze alloy, those skilled in the art will recognize that theprocess is equally well suited to forming enhanced metallurgical bonds.Therefore, for this application, unless otherwise restricted to brazing,the terms "braze alloy" and "braze joint" should be interpreted toencompass the corresponding enhanced metallurgical bonding (EMB)concepts. Enhanced metallurgical bonds use a coating or thin foil tocreate, in combination with the base metal, a liquid interface at thejoint when the EMB foil is activated at elevated temperature withsubsequent diffusion of the constituents to give common microstructureat the joint for the joint material and the base metal.

III. A COMBINED CYCLE FOR SPF AND β-ANNEALING

Another preferred combined cycle involves SPF followed by , β-annealingand is especially desirable for titanium alloys, particularly Ti-6Al-4V,that are used in aerospace products. The combined cycle actually canalso include brazing with the β-annealing step since the temperaturesneeded for these operations are corresponding or comparable. Therefore,in some circumstances the combined cycle completes three manufacturingoperations; SPF, brazing, and annealing. As with our other preferredembodiments, this combined cycle saves time, touch labor, and energy. Itproduces better parts at lower cost. The combined cycle is possibleprimarily because of the ability to heat or cool the part rapidly andcontrollably using our induction heating workcell.

Our preferred cycle is illustrated in FIG. 5, which, like FIG. 4, chartsthe time dependence of temperature with the solid line (₋₋₋₋₋₋) on ascale corresponding to the left abscissa and of pressure with thehatched line (- - - 0 - - - 0 - - - ) on a scale corresponding to theright abscissa. Time, of course, is charted on the ordinate (at thebottom) increasing from left to right from process initiation tocompletion.

In the SPF-annealing combined cycle for Ti-6Al-4V, we raise thetemperature rapidly from ambient to about 1625° F. (885° C.) at about100-120° F. /min (55-65° C./min) by heating the smart susceptor 110inductively. Then we hold the temperature substantially constant and,with the part at temperature in the SPF temperature range of the alloy,we apply pressure gradually and controllably to superplastically formthe part within the acceptable strain rate parameters, reaching aforming pressure of around 300 psi. We hold this pressure for theremaining processing steps until the part is completed with the desiredshape and microstructure, an operation that typically takes about 4hours.

After completing the superplastic forming of the part, we increase thetemperature by increasing the power fed to the induction coil with itscoil driver. When we reach the annealing temperature (about 1850° F.(1010° C.) for the β-annealing of Ti-6Al-4V ELI or 1950° F. (1165° C.)for standard Ti-6-4), we hold the temperature steady for the periodnecessary to complete the anneal. Thereafter, while maintaining thepressure over the part, we controllably cool the part to about 1200° F.at a rate of about 30-85° F./min (16-48° C. /min), which is necessary toproduce the optimum microstructure. During this cooling phase, generallywe discontinue current flow in the induction coil while containing thecirculation of cooling water. We also generally circulate pressurizedargon gas in the retort that surrounds the part. The gas also acts as acooling fluid to transport heat out of the part.

Next, we rapidly cool the part from about 1200° F. (650° C.) to about500° F. (260° C.) without concern for controlling the cooling rate nowthat the micro structure is fixed. Finally, we reheat the part byreactivating the induction coil to heat the part to a temperature ofabout 1400° F. (760° C.) where we hold the temperature steady for abouttwo hours to achieve a stabilization anneal. With, annealing completed,we rapidly cool the part, eliminating the over pressure and opening theinduction heating press after the part has cooled below about 500° F.The formed part is removed from the retort and is ready for trim, clean,and finish machining.

We maintain pressure over the part during the stabilization anneal tomaintain the desired geometry for the part.

This SPF-annealing process is particularly valuable for the manufactureof aerospace parts, and reduces complex 5-axis milling of thick titaniumblocks which is presently necessary for the manufacture of selected aftsection parts in advanced aircraft. With this SPF-annealing process, wecan machine the part with a flat geometry (FIG. 6), can then form thepart to the desired, final geometry (FIG. 7), and finally can anneal theformed part to obtain the required strength, toughness, ductility,microstructure, and other chemical and physical properties. The combinedcycle simplifies the machining/milling greatly reducing its costprincipally by reducing the required machining time. Then, the machinedintermediate is quickly, reliably, and readily formed and annealed inapproximately the same time that the necessary annealing alone wouldrequire. We save material. We simplify machining. We save time andenergy. As shown in FIGS. 6&7, the part may be machined in two piecesthat are assembled and aligned in the die. Each piece is formed into arespective individual contour such as the right and left handed complexcurvatures shown in FIG. 7 and the two pieces are bonded together toform a hollow, completed part of complex curvature.

As with our other induction heating processes, we enclose the part orworkpiece (here typically, the machined intermediate) within a retortfabricated from sealed metal sheets 110. The retort sheets function as asusceptor and are heated by the time-varying magnetic field that wecreate with the induction coil. The sealed retort sheets also permit usto surround the part with an inert gas atmosphere to prevent oxidationof the titanium and to control the microstructure during the annealing.

We manufacture the dies to conform to the desired final configuration ofthe completed part, accounting for relaxation as appropriate.

The rate and magnitude at which we apply pressure to superplasticallyform the intermediate to the appropriate final part geometry isdependent upon the part geometry. The forming time is typically on theorder of 30 minutes although the time can vary widely depending upon thepart's complexity, criticality, and geometry.

The β-annealing temperature of 1850° F. for Ti-6-4 ELI corresponds tothe melting point of the Ti--Cu--Ni braze alloy that we typically use inour multisheet titanium SPF parts, like those described in section II,so the combined cycle is conducive to combining SPF with both brazingand annealing. If we anneal, we want to control the pressure, to controlthe cooling rate to obtain the optimum microstructure, and to complete astabilization anneal, all within a single heating cycle.

While we have described preferred embodiments, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the inventive concept. The examples provided in sectionsII & III illustrate the invention and are not intended to limit it.Therefore, the description and claims should be interpreted liberallywith only such limitation as is necessary in view of the pertinent priorart.

We claim:
 1. A method for combining SPF and annealing heat cycles toimprove manufacturing efficiency by sequentially superplasticallyforming a metal part in a superplastic forming temperature range andthen annealing said part at a higher annealing temperature, comprisingthe steps of:(a) loading a pack into an induction heating press, whereinsaid pack includes said metal part enclosed within a susceptor, themetal part being capable of superplastic forming in the superplasticforming temperature range and of annealing at the annealing temperature;(b) inductively heating the susceptor to heat the part to a temperaturein the superplastic forming range but below the annealing temperature;(c) superplastically forming the heated part within the pack; (d)inductively heating the susceptor to raise the temperature of thesusceptor and, thereby, the formed part from the superplastic range tothe annealing temperature; (e) annealing the formed part at theannealing temperature within the pack; and (f) removing the pack fromthe press after completing the annealing.
 2. The method of claim 1wherein said inductively heating the susceptor to raise the temperatureinvolves increasing the power supplied to an induction coil.
 3. Themethod of claim 1 wherein the part is titanium or a titanium alloy. 4.The method of claim 3 wherein the annealing temperature is at or aboveabout 1850° F. (1010° C.).
 5. The method of claim 1 wherein the step ofsuperplastically forming includes applying a differential pressureacross the part.
 6. The method of claim 5 wherein the differentialpressure is retained until after completing the annealing.
 7. The methodof claim 1 further comprising the steps of:(g) cooling the part withinthe pack after annealing at a predetermined rate to obtain a desiredmicrostructure in the part; and (h) reheating the part inductivelywithin the pack for a stabilization anneal by holding the pack for apredetermined time at a stabilization annealing temperature below thesuperplastic forming range.
 8. The method of claim 1 wherein the partincludes multiple sheets and the method further comprises the stepsof:(i) locating a braze alloy at at least one selected location betweenadjacent sheets of the part; (j) heating the part within the pack tomelt the braze alloy; and (k) subsequently cooling the part within thepack to cool the braze alloy below its melting point to freeze the brazealloy at the selected location.
 9. The method of claim 8 wherein thebraze alloy melts at substantially the same temperature as the annealingtemperature for the part.
 10. A formed and annealed product obtained bythe method of claim
 7. 11. A product obtain by the method of claim 8wherein said multiple sheets are titanium alloy and the braze alloy isan alloy of titanium, copper, and nickel.
 12. The method of claim 1wherein the part includes at least three sheets and whereinsuperplastically forming the part includes forming a core from at leastone sheet and sandwiching the core between the other two sheets definingouter face sheets.
 13. The method of claim 8 wherein the multiple sheetsare formed into a core sandwiched by outer face sheets and wherein brazealloy is located at interfaces between the core and the face sheets. 14.The method of claim 1 wherein the part includes at least five sheets inwhich three inner sheets are formed into a core, said inner sheetshaving intermittent welds at predetermined locations to define a corecell geometry where the core is formed and wherein the core is joined tothe other two sheets defining outer face sheets by braze joints wherethe core and face sheets contact.
 15. The method of claim 3 wherein thepart is a titanium alloy, wherein annealing is β-annealing, and theannealing temperature is about 1850° F.
 16. The method of claim 9wherein the part is titanium alloy and the annealing temperature isabout 1850° F.